This invention relates to field effect transistors (FETs) and, more particularly, to vertical FETs.
In a conventional FET, the source, drain, and gate electrodes are arranged on the same major surface of a semiconductor body as depicted in FIG. 1. In general, the gate voltage controls current flow in the semiconductor channel which extends between the source and drain. The performance of an FET depends very much upon the doping profile and quality of the material proximate the surface (i.e., the active layer) and also upon the geometry of the device.
In some applications, e.g., where high power capability is desired, the FETs are connected in parallel with one another. Because all three electrodes are located on the same surface, relatively complicated crossover metallization patterns are required to effect the parallel connections. Elimination of this problem would facilitate large scale integration of FETs.
The geometry of the FET also gives rise to another problem. The gate width W.sub.g (FIG. 1) is very large compared with the gate length L.sub.g. Therefore, the gate may be viewed as a transmission line terminated in an open circuit load. A signal impressed at the gate pad is propagated down the long narrow strip of the gate electrode where it experiences attenuation and reflection. As a consequence, the voltage along the gate electrode is different at different sections, and the overall FET may be approximated as many small sections of FETs operating in parallel. Using this approximation, it can be shown that the noise figure of the FET is linearly proportional to the gate length L.sub.g. However, state-of-the-art photolithographic fabrication techniques can achieve dimensions only of the order of 1 .mu.m. Smaller dimensions are less reproducible and encounter problems of diffraction and proximity effects. Alternative fabrication techniques, such as X-ray or electron beam exposure, realize smaller dimensions of 0.2 .mu.m, but the resulting high current density in the electrode may cause electromigration problems.
One device suggested in the prior art which might alleviate these problems is known as a "vertical" FET; that is, an FET in which the channel extends vertically and transverse to the active layer of the device rather than horizontally and parallel to that layer. This change in channel orientation can be achieved in different ways. J. G. Oakes et al, IEEE Transactions on Microwave Theory, Vol. MTT-24, No. 6, pages 305-311 (1976), fabricated a mesa geometry, vertical MOSFET using an angle evaporation shadow technique to position the gate electrode on the sides of a silicon mesa. The drain electrode was formed on the bottom of the substrate; the source electrode on the top of epitaxial layers grown on the substrate. The effective gate length (on the order of 1 .mu.m) was measured by the thickness of the epitaxial active layer. Because all three electrodes were not formed on the same surface of the device, in one sense parallel interconnection of a plurality of FETs would be facilitated, but in another sense the nonplanar geometry might seriously complicate electrode fabrication.
In contrast, D. L. Lecrosnier et al, IEEE Transactions on Electron Devices, Vol. ED-21, No. 1 (1974), utilized high energy ion implantation coupled with planar technology to fabricate a "Gridistor", a vertical multichannel, silicon FET with a p-type buried gate. The gate and source contacts were located on the top major surface of the epitaxial layers whereas the drain was on the bottom of the substrate. These FETs were characterized by a low figure of merit and high gate-to-source capacitance due, in part, to lack of sufficient control (lateral spreading) of implanted boron ions. The asymmetrical distribution of the boron ions also placed a lower limit on the thickness of the buried gate layers.